1. Field of the Invention
This invention generally relates to a lightning arrestor mounted to an electric transmission tower, more particularly to a lightning arrestor having a series gap.
2. Description of the Related Art
A lightning arrestor design having a series gap is commonly used to prevent a grounding fault of overhead transmission line due to the lightning surge. Such arrestors accommodate a plurality of zinc oxide element segments having non-linear voltage-current characteristics. The arrestor unit is connected in parallel with an insulator by way of an aerial discharge gap.
In the conventional arrestor mounted to a double-circuit electric transmission system, the arrestor have been applied only in the single circuit for the purposes both to prevent double circuit faults and to minimize the installation cost. In such transmission lines, however, the lightning strike causes a grounding fault on the circuit in which the arrestor is not installed. The ground fault causes an increase in the nominal line to ground voltage E of the other circuit carrying the arrestor. It is assumed that the ground fault causes a voltage increase of up to the voltage of .sqroot.3.E in case of non-effective grounding system. Since it is required for the arrestor to be operated when the line voltage is .sqroot.3.E, the reference voltage or the critical operating voltage of the arrestor unit should be at least .sqroot.3.E. The length of arrestor unit is determined by the rated voltage, that is the number of zinc oxide block is determined by the increased line to ground voltage E.
However, such an arrestor unit having a rated voltage of .sqroot.3.E includes a rather large number of arrestor elements for safely absorbing the lightning surge. Thus, the resultant arrestor is not compact and economical.
Furthermore, the insulating level or flashover voltage due to the lightning surge should be kept sufficiently lower than that of the insulator to reliably absorb the lightning surge in the arrestor. The lightning surge flashover voltage in the arrestor is the sum of the lightning surge flashover voltage in the aerial discharge gap plus the bias voltage in the arrestor elements. This bias voltage is generally in proportion to the reference voltage or critical operating voltage. Thus, when the number of arrestor element segments increases, the reference voltage becomes higher in accordance therewith. This effectively becomes a limitation when trying to lowering the insulating level of the arrestor unit. Especially, when the arrestor is mounted to the tower carrying a small number of insulators, it is difficult to obtain a sufficient insulation co-ordination between the circuit lines as well as between the arrestor unit and insulator, causing the insulating levels being relatively close to each other. This results in a disadvantage of the arrestor whereby the lightning surge is not reliably absorb to perfectly prevent ground faulting.
Further, in the event that the arrestor is mounted to a suspension tower, the discharge electrode tends move due to swinging of the lines in the wind. This varies the length of the discharge gap. The extension of the discharge gap makes it impossible to obtain the sufficient insulation co-ordination, causing the frequent grounding faults. Therefore, the conventional gapped type arrestor requires an extended discharge electrode with a complicated structure in order to keep the discharge gap at a predetermined length.
When studying the above problems in the conventional art, the present inventor became aware that an arrestor having the arrestor elements of which the rated voltage is less than .sqroot.3.E is still able to absorb the lightening induced surge without being damaged. At the time of a lightening strike, it is very rarely necessary for the arrestor to absorb the lightning surge with a voltage as high as .sqroot.3.E.